Gros, U.S. Pat. No. 3,426,063, discloses a process for inhibiting thermal polymerization and/or the growth of popcorn polymer in compositions containing polymerizable ethylenically unsaturated hydrocarbons and/or esters in which there is admixed therein an N-nitrosoarylhydroxylamine or, preferably, a salt thereof. N-nitrosophenylhydroxylamine is among the specific compounds which are disclosed to be especially preferred. Examples given of salts of the N-nitrosoarylhydroxylamine which may be employed include the ammonium salts and, in some instances, "organic amine salts of the acidic tautomeric form of the N-nitrosoarylhydroxylamine . . . , the organic radicals of which each may contain 1-20 and preferably 8-16 carbon atoms." Gros discloses that, usually, the ammonium salts are water soluble and thus may be preferred for use in aqueous systems; while, on the other hand, unspecified "certain amine salts" are very soluble in the hydrocarbon monomer and thus may be preferred for use in all-monomer systems or organic solvent solutions of monomers. Gros specifically discloses that an organic solvent solution of cupferron (the ammonium salt of N-nitrosophenylhydroxylamine) or "the" amine salt of N-nitrosophenylhydroxylamine ma be added to an all-monomer system.
Although cupferron is an effective polymerization inhibitor, it has not been entirely satisfactory from a number of standpoints, including its limited solubility and limited storage stability. Cupferron is typically added to monomers such as acrylic acid in miniscule amounts (e.g. from about 20 to 200 parts per million (ppm) parts of monomer). To facilitate control over the addition in such miniscule amounts, users generally prefer to add cupferron as a liquid solution thereof in an amount and type of solvent which are acceptable for a given use. The acceptable amounts and types of solvent are dependent upon the particular application.
Although water is an acceptable solvent for introducing cupferron into systems containing acrylic acid where water is either present or permissible, water is not acceptable in appreciable amounts in final distillation steps in the manufacture of glacial acrylic acid nor in acrylate esters.
Acrylate esters may have acceptably introduced into them an alcohol, especially the particular alcohol from which the ester is derived, and accordingly an alcohol may be employed as solvent for adding cupferron to such esters in some instances. However alcohols, including the alcohol from which the ester is derived, are generally undesirable in the final purification stages of such esters due to the risk of co-distillation of the ester and the alcohol and resulting product contamination. It is generally unacceptable to admix methanol or other alcohols with acrylic acid in industrial facilities for the manufacture thereof due to the risk of esterification of the acid.
In general, users of cupferron require liquid solutions thereof in which the ratio of solvent to cupferron is low, e.g. 10:1 or less. Achieving a low ratio requires a solvent in which cupferron is highly soluble, e.g. in an amount of at least 10% for the ratio not to exceed 10:1. All parts, percents and other amounts setforth herein are by weight, unless otherwise indicated. Ratios lower than 10:1 (requiring solubilities of more than 10%) would be preferable.
Unfortunately, cupferron has limited solubility in selectively acceptable solvents such as water and lower alcohols (e.g. methanol etc.). Cupferron has the following limits of solubility at 25.degree. C. in the indicated solvents: water (12%), methanol (5.5%) and isopropanol (0.38%), while it is essentially insoluble in hydrophobic solvents i.e., solvents which are immiscible with water). As a result of the limited solubilities of cupferron in the foregoing solvents therefor, objectionably large amounts of these solvents are required in many applications where cupferron would otherwise be a desirable inhibitor.
Solutions of cupferron, e.g. aqueous solutions thereof, are not entirely stable and undergo degradation in the presence of air, as manifested by discoloration and formation of a black precipitate. Minimizing the extent and rate of degradation requires, in practice, storing such solutions under an inert atmosphere such as nitrogen.
The various above-mentioned deficiencies of cupferron solutions are not overcome by the organic amine salts of N-nitrosoarylhydroxylamine disclosed in the above-cited Gros patent. On the contrary, organic aliphatic amine salts of N-nitrosophenylhydroxylamine (hereinafter sometimes referred to as NPHA) are unsatisfactory for use in inhibiting polymer formation in acrylic acids and acrylate esters. In the presence of such monomers, the amine salts decompose with formation of the organic aliphatic amines (e.g. ethylamine, etc.). The lower boiling amines (e.g. the C.sub.1 to C.sub.7 aliphatic amines) formed by decomposition of the corresponding amine salts create a substantial risk of co-distillation thereof with the acrylic acid or acrylate ester monomers being purified by distillation and resulting discoloration of polymers prepared from the monomers, such as, for example, poly(acrylic acid), poly(methyl methacrylate) and poly(ethyl acrylate). The higher boiling amines (e.g. the C.sub.8 -C.sub.20 aliphatic amines) formed upon decomposition of the corresponding amine salts are so immiscible with water that such salts are not entirely satisfactory for addition to water-containing acrylic acid systems.
Accordingly there is a substantial need in the art for a salt of N-nitrosophenylhydroxylamine which is both (a) highly soluble in solvents which can suitably be added to acrylic acid and acrylate ester systems and (b) effective for inhibiting undesired formation of polymer in such systems.